Leaching of precious metals from precious metal-containing ores or concentrates is a common commercial way for the production of precious metals. The cyanidation process is the current industry standard for leaching of precious metals. During cyanidation of precious metals, particularly gold, the metals are recovered from milled ores containing such metals by contacting with an alkaline cyanide-containing solution. With the formation of cyano complexes, precious metals are leached into solution for separation and later recovered by either electrowinning or zinc dust. In the case of gold, the dissolution of gold by cyanidation leaching can be represented as:4Au+8NaCN+O2+2H2O→4Na[Au(CN)2]+4NaOH
A large number of ores may, however, contain various amount of carbonaceous material, from 0.1% to up to 10%. Investigations on these kinds of ores suggested the carbonaceous materials are mostly naturally active carbon and long-chain hydrocarbons. Numerous studies indicated that presence of carbonaceous material in gold-bearing ores decreases the cyanidation leaching efficiency and the corresponding gold recovery. The interference of carbonaceous material with cyanide leaching may occur through formation of stable complexes by carbonaceous material with the gold or lock-up of gold within carbonaceous material, or adsorption of aurocyanide from cyanide leaching solution by the naturally active carbon on the ores; the latter is generally a more common problem with carbonaceous ores. When the interference occurs, some portion of the gold in the carbonaceous ores will not be available either for leaching and recovery from solution.
Roasting of carbonaceous ores at temperatures above 1000 F may effectively take away most of the detrimental effects of carbonaceous material on cyanidation leaching, however, performing such a step significantly increases energy cost and is faced with tight environmental regulations. Leaching of carbonaceous ores by thiosulfate has been suggested, in which carbonaceous material could have decreased interference with the leaching efficiency and the corresponding gold recovery.
Several ways to treat carbonaceous ores by different chemicals have been disclosed. For example, a process of using fatty acid salt for cyanidation of carbonaceous ores is known. Further, the cyanidation of carbonaceous ores using oleaginous substance and also an anionic soap substance is also known. Furthermore, the use of wetting agent of anionic sulfonate/sulfate, sulfosuccinate, and naphthalene sulfonate in the cyanidation of carbonaceous and non-carbonaceous ores has been suggested. A bioleaching process to recover precious metals from carbonaceous and non-carbonaceous ores after treatment of the ores with a plant-derived aromatic component has also been suggested.
A study has been done to examine the effects of surface active agents on gold (specifically gold cyanide) adsorption by (Ghana) carbonaceous gold ore, with pre-treating ores by surface active agents at pH 10. The examined surface active agents included: DDA (dodecylamine with alkyl chain of 12) and Aliquat 336 (tricaprylylmethylammonium chloride with alkyl chain length of 8 and 10). In the study, no cyanide leaching test was done. The report of the study indicated that gold uptake in aqueous cyanide solution is enhanced markedly by Aliquat 336 and slightly by DDA; this implied that detrimental effects of reduced gold recovery would occur if cyanide leaching test was ever done on the ore. In stark contrast, cyanide leaching results on carbonaceous gold ores of the present invention (as further shown below) show beneficial effects of increased gold recovery when the ores were pre-treated with cationic surface active agents having longer alkyl chain length, such as fatty amine and their derivatives with alkyl chain length from 14 to 40.
A method for treating copper-containing gold ores by surface active agents is also known. This method, however, concerns the presence of other metals in the gold ores which are liable to form cyanide salts, particularly copper (chalcopyrite). It is well-known that metallic minerals such as chalcopyrite or pyrite are preg-robbing (K. L. Rees et al., “Preg-robbing phenomena in the cyanidation of sulphide gold ores,” Hydrometallurgy 58, 2000, pp. 61-80, stating that “chalcopyrite was shown to be very strongly preg-robbing. It competed with activated carbon to remove the majority of gold from solution. Pyrite was also strongly preg-robbing”). This known method for treating copper-containing gold ores was based on the finding that the deleterious effect of copper-containing minerals could be reduced by a form of passivation pre-treatment during or prior to cyanidation. The copper was not removed but passed through the cyanidation process with a reduced tendency to form cyanide complexes.
Hence, there is still a need for a method of treating carbonaceous material containing precious metal ores or concentrates which provides increased recovery of the precious metal.